The present invention relates to techniques for estimating a physiological parameter from a physiological signal. More specifically, the invention relates to detecting and estimating oximetry signals from physiological signals which include noise, and even more specifically to techniques for accurately determining the pulse rate from noisy physiological signals.
It is well known that physiological parameters (e.g. blood oxygen saturation and pulse rate) are represented by physiological signals, and that such signals often contain substantial noise components, often much larger than the physiological signal component. For example, the blood oxygen saturation (SpO2) level in the blood stream may be determined by shining red and infrared (IR) light on a blood perfused part of a patient""s body (e.g. finger or earlobe). The light passing through, or reflected off, the patient is detected and signals representing the received light are generated. These signals are then processed to generate both an indication of the pulse rate and the blood oxygen level of the patient.
One problem with such systems is a noise component in the light representative signals. This noise component is substantially caused by movement by the patient, however electromagnetic interference from surrounding equipment, and reception of ambient light by the light sensors also contribute to the noise component. In some cases, this noise component can be substantially large, compared to the signal component. Systems were designed to detect the signal components in the light representative signals in the presence of a relatively large noise component.
Recently, techniques using fast Fourier transforms (FFT) of the light representative signals have been developed. In U.S. Pat. No. 5,632,272, issued May 27, 1997 to Diab et al., data from an FFT of the light representative signals is analyzed to determine the arterial blood saturation. In this patent information from all the FFT frequencies above a threshold level is analyzed with equal weight.
In U.S. Pat. No. 6,094,592, issued Jul. 25, 2000 to Yorkey et al., generates a ratio signal having a value corresponding to each frequency location in the FFT spectrum, then generates a histogram of the values of the ratio signal weighted by the magnitude of the IR FFT at the frequency associated with the ratio value.
In all of these systems, the FFT signal was processed according to an algorithm and a pulse rate signal and SPO2 signal generated. However, there are always clinical situations in which a particular algorithm will perform poorly, and conversely other clinical situations in which that algorithm will perform well. A system which can operate optimally over a range of different clinical situations is desirable.
In accordance with principles of the present invention, a system first identifies a plurality of characteristics of a physiological signal any one of which may represent a physiological parameter. A plurality of different techniques are used to provide respective likelihood factors for each such identified characteristic. The resulting likelihood factors are then analyzed to select the one characteristic of the physiological signal which most likely represents the desired physiological parameter. The physiological parameter is then calculated based on the selected characteristic of the physiological signal.
More specifically, a system according to principles of the present invention determines the parameter of pulse rate from SpO2 physiological signals, which include red and IR light representative signals. The frequency locations of peaks in the spectrum of the IR light representative signal are detected as the characteristics. A plurality of different techniques each generate a likelihood factor for each identified peak, respectively. All of the likelihood factors are then analyzed to select one of the identified peaks as the characteristic most likely representing the actual pulse rate. The pulse rate parameter is calculated from the red and IR light representative signals at the frequency location of the selected peak. The red and IR signals at the frequency location of the selected peak may be further processed to generate a second parameter of the SpO2 value.